This invention relates to a projection-type display apparatus such as a liquid-crystal projector.
A structure of an optical system of a conventional liquid-crystal projector is explained below with reference to FIG. 8. In this figure, the light emitted from the lamp 1 is reflected by the reflecting mirror 2 to become a collimated light beam 3. This collimated light beam 3 is not uniform in luminance distribution within its cross section normal to the direction of travel (optical path), which will cause a projected image to have unevenness in brightness if the beam 3 is used as it is. Therefore, the beam 3 is caused to pass through the multilens arrays 4, 5 each of which is comprised of small convex lenses that have been cut to have a rectangular profile, so that the beam has uniform luminance distribution within its cross section when illuminating a liquid crystal panel.
To be more specific, the collimated light beam 3 enters the multilens array 4, and is split into a plurality of light beams by its small convex lenses. Each of the splitbeams enters a corresponding small convex lens of the multilens array 5, and is led to the polarization-conversion device 6 to be converted into a linearly-polarized light beam. The linearly-polarized light beam exits from the condenser lens 7 as a beam converging on the plane of incidence of the liquid crystal panel. The beam emitted from the condenser lens 7 is folded by 90 degrees in its optical path by the mirror 8, and enters the dichroic mirror 20. The dichroic mirror 20 allows the red light beam to pass, and reflects the blue and green light beams.
The red light beam that has passed through the, dichroic mirror 22 is folded by 90 degrees in its optical path by the reflecting mirror 22 to illuminate the liquid crystal panel 10R after its converging angle is corrected by the collimator lens 9R.
On the other hand, the blue and green light beams reflected from the dichroic mirror 20 enter the dichroic mirror 21. The dichroic mirror 21 allows the blue light beam to pass, and reflects the green light beam. The green light beam reflected from the dichroic mirror 21 illuminates the liquid crystal panel 10G after its converging angle is corrected by the collimator lens 9G. The blue light beam that has passed through the dichroic mirror 21 is folded by 180 degrees in its optical path by the reflecting mirrors 23, 24 and the relay lens 15, 16 to illuminate the liquid crystal panel 10B after its converging angle is corrected by the collimator lens 9B.
The liquid crystal panel 10R modulates the incident light beam in accordance with an R-video signal, that is, allows only selected parts of the incident light beam to pass to form a red image. The red light beam that has passed through the liquid crystal panel 10R enters the dichroic prism 11, and is folded in its optical path to go into the projection lens 12. The liquid crystal panel 10G modulates the incident light beam in accordance with a G-video signal, that is, allows only selected parts of the incident light beam to pass to form a green image. The green light beam that has passed through the liquid crystal panel 10G enters the dichroic prism 11, and goes into the projection lens 12 directly. Likewise, the liquid crystal panel 10B modulates the incident light beam in accordance with a B-video signal, that is, allows only selected parts of the incident light beam to pass to form a blue image.
The blue light beam that has passed through the liquid crystal panel 10B enters the dichroic prism 11, and is folded in its optical path to go into the projection lens 12. The light beams of the red, green and blue images incident upon the projection lens 12 are combined and projected as a full-color image.
FIG. 9 shows a part of the optical system having the above-described structure. Shown in this figure is an optical path from the lamp 1 to the projection lens 12 via the liquid crystal panel 10G. However, the mirrors for changing the directions of the beams and the dichroic prism 11, etc. are omitted.
The light emitted from the lamp 1 is reflected by the reflecting mirror 2 to become a collimated light beam 3. The collimated light beam 3 having nonuniform luminance distribution within its cross section normal to the optical path are split into a plurality of light beams by the multilens array 4. The multilens array 4 focuses the light beams towards the multilens array 5. The multilens array 5 maintains the multilens array 4 and the liquid crystal panel 10 in a conjugate relation. A plurality of the light beams that have passed through the multilens array 5 enter the polarization-conversion device 6 and are split into two linearly-polarized light beams having mutually orthogonal vibration planes.
The vibration plane of one of the two linearly-polarized light beams is rotated by 90 degrees by the phase plate 13 disposed at the output plane of the polarization-conversion device 6. In consequence, all the light beams are converted into one linearly-polarized light beam having an identical vibration plane before entering the condenser lens 7. The condenser lens 7 causes the images of the small convex lenses of the multilens array 4 that are formed on the multilens array 5 to overlap with each other on the plane of incidence of the liquid crystal panel 10 in order to produce an illuminating light beam having a rectangular cross section within which luminance distribution is uniform. The collimator lens 9 disposed before the liquid crystal panel 10 uniforms the converging angles of the light beams that enter the liquid crystal panel 10.
The lamp 1 has a directivity as shown by the broken line in FIG. 10, and accordingly emits the light largely to the directions perpendicular to the optical axis. Therefore, the reflecting mirror 2 is necessary to lead the light toward the front along the optical axis and let the light in the optical system efficiently. The reflecting mirror 2 has a paroboloidal inner surface to reflect the light emitted from the lamp 1 so that it makes a collimated light beam.
In order to let the light incident from the lamp 1 in the optical system as much as possible to increase the efficiency of use of the light, it is necessary for the reflecting mirror 2 to have a large diameter D. In addition, in order to lead the light from the reflecting mirror 2 to the liquid crystal panel efficiently, it is necessary to make the areas of the optical devices such as the multilens array 4, the multilens array 5, the polarization-conversion device 6, and the condenser lens 7 about the same as the area of the opening of the reflecting mirror 2.
Accordingly, the size of the optical system of such a conventional projection-type display apparatus greatly depends on the size of its reflecting mirror for collimating the light emitted from its light source. Therefore, there has been a problem that when the size of the reflecting mirror is increased to improve the efficiency of use of the light, the optical system has to be upsized for that, and the apparatus is upsized as a consequence.
An object of the present invention is to increase the efficiency of use of the light without upsizing the optical system in a projection-type display apparatus.
This object is achieved by a projection-type display apparatus including a light source, an integrator whose input plane receives a light emitted from the light source and whose output plane serves as a surface illuminant, an image forming device modulating an incident linearly-polarized light beam to form an image in accordance with a video signal, a polarization-conversion device converting a uniform light beam exiting from the integrator into a linearly-polarized light beam to be applied to the image forming device, and a projection device projecting the image formed by the image forming device. The light source is constituted by a light emitting diode.
According to the present invention, a compact and lightweight projection-type display apparatus capable of displaying bright images can be provided.